Now available at Precision Aero Products are a number of items from SM Modellbau of Germany. One such product is the Unisens-E telemetry sensor. The sensor is about the size of a 20 cent piece and measures an array of onboard parameters and passes them back down to your transmitter.

The sensor works with all popular brands of radio and is programmable via a USB interface cable (sold seperately) and the free PC software application. Once you use one of these sensors you'll wonder what you did without it... Look here for more information and pricing.